An inter-device session (IDS) encompasses scenarios where two or more devices directly communicate data through a shared radio channel. For a given IDS resource allocation, one User Equipment (UE) device in the session transmits data in the allotted resource, and another UE in the session receives data in that allotted resource. An associate wireless network may allocate the IDS resource from uplink (UL) or from downlink (DL) resources or from both.
In a Long Term Evolution (LTE) system, an evolved Node B (eNB) allocates resources for the IDS to UEs. Using the allocated resources, the UEs directly transmit data traffic for the IDS, and the eNB may control operational aspects of the IDS. For example, the eNB may transmit, to the UEs, control information using a Physical Downlink Control Channel (PDCCH), or other downlink signal. The control information can include a resource allocation, a Modulation and Coding Scheme (MCS) for traffic, a power control command, or other information.
In LTE DL transmissions, the data of different UEs are multiplexed in the frequency domain (subcarriers), and a single OFDM symbol is formed by applying an inverse fast Fourier transform (IFFT) across all subcarriers and appending a cyclic prefix (CP). At the receiver (UE) side, the CP of the OFDM symbol is discarded and FFT is applied to the remaining portion of the received symbol to convert the data from the time domain to the frequency domain. The UE demultiplexes the converted data in the frequency domain. The time-window over which the FFT is applied should be aligned with the received OFDM symbol to avoid inter-symbol interference (ISI) and inter-carrier interference (ICI). Each UE may execute time-alignment (or time-synchronization) to DL signals from the eNB in an open loop manner using the synchronization signals that the eNB broadcasts.
Time-alignment for UL signals is different than for DL signals. At the receiver side for UL signals, i.e., at eNB, OFDM symbols from multiple UEs are superimposed and processed at the same time. Although different UEs are orthogonal in the frequency domain, for the receiver to be able to detect all UEs' data with a single FFT operation, all the OFDM symbols should arrive at the eNB approximately time-aligned. Misalignment of different OFDM symbols can cause ICI and performance degradation. Time-alignment for UL signals in LTE systems is carried out in a closed loop manner. At initial access to the LTE system, which uses a physical random access channel (PRACH), each UE's transmission is based on the DL timing acquired through DL synchronization signal. Due to a round-trip delay, the initial transmission of the UE arrives with some delay relative to UL timing for the eNB. The eNB measures this delay based on the PRACH signal received from the UE and transmits a timing advance command to the UE to advance UL transmissions by a certain amount of time. The timing advance is an integer multiple of 16Ts (or about 0.52 μsec), where Ts is the sampling period of the OFDM system. Using the timing advance command, the UE corrects the transmission timing and the next transmission arrives at the eNB approximately aligned with the UL symbol timing of the eNB. As the UE may move and the propagation delay may change, the time alignment procedure periodically repeats. After the initial access using PRACH, the delay may be later measured using PRACH signals or the SRS.